Gates and Branson use their notebooks to record ideas. By keeping a pen and paper handy, these two moguls are able to keep track of and remember all the inklings that come to them throughout the day.

"I can't tell you where I'd be if I hadn't had a pen on hand to write down my ideas as soon as they came to me," Branson wrote in a blog post.

Sandberg uses her notebook as a sort of daily planner, according to Fortune. She scribbles down to-do lists and then rips out pages once she's hit all the items on her list. It's a great motivator for staying on track — there's nothing more satisfying than crossing off everything on your to-do list.

Regardless of how exactly they use their notebooks, these business leaders are all onto something.

TED speaker Damon Brown wrote in Inc. that writing things down helps us filter our thoughts, remember ideas and insights, and articulate the abstract.

Forbes contributor Drew Hansen says that the key is to always jot down your thoughts straight away: "You think you'll remember, but you won't, and you'll forfeit all the thoughts that flood you after you've freed your mind from remembering the initial spark."